Generally the method for forming a multi-layer paint film when the object being painted is an automobile involves forming an electro-deposited paint film on the part which is to be painted and curing this by heating and then forming a multi-layer pain film comprising a mid-coat paint film, a colored base paint film and a clear paint film. Here the clear paint film provides various characteristics such as acid resistance, water resistance, weather resistance and the like and not just an excellent paint film appearance.
Generally an acrylic resin which has excellent water resistance, resistance to chemical attack, weather resistance and the like is used as the main resin of a clear coat paint, but clear paint compositions with which clear coat paint films which have excellent characteristics can be obtained by using polyester resins are also known.
A film-forming binder which includes polyester resin which has a specified alicyclic part content and which has specified characteristic values has been disclosed in Patent Document 1 as a clear paint composition in which polyester resin is used.
Furthermore, a coating composition which includes a binder which includes polyester resin which has specified characteristic values, acrylic polyol which has specified characteristic values and crosslinking agent which reacts with hydroxyl groups has been disclosed in Patent Document 2.
However, with the paints disclosed in Patent Document 1 and Patent Document 2 there is a problem in that a good paint film appearance cannot be obtained with the so-called 3C1B type multi-layer paint film forming method which includes a first base paint film forming process in which a first base paint is coated on the object which is being painted and a first base paint film is formed, a second base paint film forming process in which a second base paint is coated over the uncured aforementioned first base paint film and a second base paint film is formed, a clear paint film forming process in which a clear paint is coated over the uncured aforementioned second base paint film and a clear paint film is formed and a heating and curing process in which these three paint film layers are heated and cured at the same time.
Moreover, a hydroxy-functional binding agent which has specified characteristic values has been disclosed in Patent Document 3. In this patent document a polyester resin can be used for the hydroxy-functional binding agent and those where at least one hydroxyl group of the polyester resin has been esterified with at least one non-cyclic aliphatic mono-carboxylic acid are especially useful.
Furthermore a paint composition which contains as essential components a carboxyl group-containing polyester resin, carboxyl group-containing radical copolymer, an epoxy group-containing radical copolymer and a crosslinking agent which reacts with hydroxyl groups has been disclosed in Patent Document 4. In this patent document carboxyl group-containing polyester resins obtained by reacting specified components which have six or more carbon atoms with a polyol and then carrying out an addition reaction with an acid anhydride are preferred and, furthermore, a polyester resin can be used for the polyol.
However, with the inventions disclosed in Patent Document 3 and Patent Document 4 again a good paint film appearance is not obtained with a multi-layer paint film forming method with the 3C1B system and, furthermore, it is not possible to obtain paint compositions which have good storage stability under high temperature conditions.